The present invention generally relates to packages and packaging methods for electronic components. More particularly, this invention relates to a package and method for protecting a shock-sensitive electronic component, such as motion sensors that include micromachined structures.
Electronic packages are widely employed to protect electronic components from hazards posed by their intended operating environments, including heat, moisture condensation, chemical attack, vibration, shock, impact, etc. Notable examples are motion sensors, which include gyroscopes and their components (e.g., angular (yaw) rate sensors and accelerometers), used in aerospace and automotive safety control systems and navigational systems and in consumer goods such as VCR cameras. Examples of automotive applications include anti-lock braking systems (ABS), active suspension systems, supplemental inflatable restraint (SIR) systems such as air bags and seat belt lock-up systems, and crash sensing systems.
Yaw rate sensors of the type used in ABS and SIR systems are typically located in the passenger or engine compartment and employ one or more acceleration sensing elements, such as a micromachined structure formed in or on a silicon chip. The chip is typically mounted on a printed circuit board that is secured in an open case and sealed with a potting material that protects the circuit board, its sensing element, and any other components within the case from the hostile environment of the engine compartment. The vehicle manufacturer often specifies the environment and test requirements that the resulting package must survive. One such test is a 1 to 1.5 meter drop test performed on a completed package to verify that the package can withstand an accidental drop during vehicle installation. An additional requirement of the package is to faithfully and reliably transmit the acceleration of the case to the sensing element within the case. The desire for motion sensing packages to survive greater impacts has given rise to a conflict between the requirement to accurately transmit acceleration to the sensing element while also providing a degree of shock resistance, particularly in the case where the sensing element is a relatively fragile micromachined structure.
While existing packaging systems have proven to be extremely reliable in numerous applications, it would be desirable if an electronic package were available that further improved the shock resistance of an electronic component within the package, yet was capable of faithfully and reliably transmitting acceleration to the component, particularly when the component is a mechanical motion sensing element.
The present invention provides an electronic package and a method for packaging an electronic component, particularly a shock-sensitive component. Specific electronic components of interest are yaw rate sensors and accelerometers that are mounted to a circuit board and employ a mechanical motion sensing element, such as a micromachined structure. The package of this invention includes a case having an opening through which the circuit board is placed within the case, so that a peripheral edge of the circuit board is adjacent but spaced apart from a wall of the case. A thixotropic gel is deposited in the space between the peripheral edge of the circuit board and the wall of the case, preferably before the board is installed, so as to separate and to control the degree of mechanical decoupling of the circuit board to the case. As a result, the gel significantly reduces the amount of shock that the circuit board and its component(s) are subjected to during the sharp acceleration/deceleration associated with an impact (e.g., a dropped package during vehicle assembly).
In the method of this invention, placement of the gel and circuit board within the case is preferably facilitated by forming the wall of the case to define a shoulder and a surrounding wall portion that provide a receptacle for the gel along the wall of the case. The circuit board is then installed so that its peripheral edge contacts the gel, which spaces the circuit board from the shoulder and the wall portion. Also in the preferred embodiment, the circuit board defines two cavities within the case. A first of the cavities is defined between the board and the opening of the case, while the second is an enclosed cavity between the board and the case in which the electronic component is housed. An elastomeric material is deposited in the first cavity to encapsulate and secure the circuit board within the case. The thixotropic gel seals the board-case interface to prevent the elastomeric material from intruding into the second case.
As noted above, the package and packaging method of this invention are particularly suitable for protecting yaw rate sensors and accelerometers that employ a micromachined motion sensing element. According to the invention, the package provides improved shock resistance for a motion sensor within the case, yet enables acceleration and motion to be faithfully and reliably transmitted to the sensing element of the sensor. Suitable compositions for the thixotropic gel can be accurately dispensed in the case with minimal waste and subsequently cured at room temperature, thereby minimizing processing costs. An additional advantage is the flexibility of the packaging system in terms of the ability for the thixotropic gel to be used with cases of different shapes and sizes, avoiding the necessity for additional inventory requirements, tooling changes, capital investment and logistic costs that would be associated with the use of a preshaped material.
Other objects and advantages of this invention will be better appreciated from the following detailed description.